Its actually very much a personal thing I think - what is one persons "good" could be another persons "excellent", one persons "bait" could be another persons "poor". In many ways for differentiating between the descriptions it depends what you consider a fault or not a fault - there is not any set guide as to what fault brings a pony down a level or what issues are accepted as normal and therefore classes a pony in a higher bracket.
I am not a massive fan of simply classifying a pony into a condition bracket because sometimes it can be misleading - if I call a pony in near mint condition but she has a small scuff to her blush and a tiny scratch to her symbol who is to say another collector may not consider those issues significant faults and technically call her good.
I think for sites like ponydex and generally listing the condition of your ponies, you are best setting your own list and what you consider acceptable for each grade. For example for me mint means nothing wrong, not a hair out of place or a single mark - literally the perfect pony [which probably doesnt exist lol] but for other people mint could mean as perfect as possible but not immaculate.
As NoDivision says if your ponies are fresh off their cards then they "should" be close to mint but due to the general quality control of G4 if you are describing them as mint that would probably have to mean without any hair issues or plastic denting or scratches or head/body mismatch you may want to check them over before you list them.